This invention relates to a CDMA receiver and, more particularly, to a CDMA receiver for applying despread processing to direct waves or delayed waves that arrive via each path of multiple paths, applying synchronous detection processing to the despread signals obtained, combining the detection signals of respective paths and discriminating the received data on the basis of the combined signal.
DS-CDMA (Direct Sequence Code Division Multiple Access) communication is the focus of attention as a promising candidate for next-generation wireless access because this scheme makes possible high-speed data communication for voice, facsimile, electronic mail, still images and moving images. Such a DS-CDMA communication scheme achieves spectrum spreading by directly multiplying a signal (transmit information), which is to undergo spectrum spreading, by a signal (a spreading code) having a band much broader than that of the first-mentioned signal.
In mobile communication, maximum frequency is decided by the velocity of the mobile station and the frequency of the carrier waves, random changes in amplitude and phase occur and so does fading. As a consequence, it is very difficult to achieve stable reception at maximum frequency in comparison with stationary radio communication. The above-mentioned spread-spectrum communication scheme is effective as means for mitigating such deterioration caused by the influence of frequency-selective fading. The reason for this is that since a narrow-band signal is spread over a high-frequency band and then transmitted. This means that even if a decline in reception field strength occurs in a certain specific frequency region, information from elsewhere in the band can be reconstructed with little error.
Further, with mobile communication, delayed waves from distant high-rise buildings or mountains give rise to a multipath fading environment if fading similar to that mentioned above is produced by the receiver surroundings. In the case of direct sequence (DS), the delayed waves constitute interference with respect to the spreading code and invite a decline in reception characteristics. RAKE reception is known as one method in which such delayed waves are used positively in improvement of characteristics. RAKE reception involves subjecting each delayed wave that arrives via each path of multipath to despread processing using a code identical with the spreading code, subjecting the obtained despread signals to delay processing conforming to the path to thereby make the timings agree, subsequently performing synchronous detection, combining signals, which are obtained by such synchronous detection, upon applying weighting in accordance with the reception level, and discriminating data using the combined signal.
FIG. 28 is a diagram showing the structure of a CDMA receiver. A radio unit 1 converts a high-frequency signal received by an antenna to a baseband signal by applying a frequency conversion (RF→IF conversion). A quadrature detector 2 subjects the baseband signal to quadrature detection and outputs in-phase component (I-phase component) data and quadrature component (Q-component) data. The quadrature detector 2 includes a receive-carrier generator 2a, a phase shifter 2b for shifting the phase of the receive carrier by π/2, and multipliers 2c, 2d for multiplying the baseband signal by the receive carrier and outputting the I-component signal and the Q-component signal. Low-pass filters (LPF) 3a, 3b limit the bands of these output signals and AD converters 4a, 4b convert the I- and Q-component signals to digital signals and input the digital signals to a searcher 5, with performs a multipath search, and to a RAKE receiver 6. The RAKE receiver 6 has a plurality fingers 6a, to 6a, for executing processing (despreading, delay-time adjustment, synchronous detection, etc) conforming to the respective paths of multipath, a RAKE combiner 6b for weighting the signals, which are output from the fingers, in accordance with the reception level and then combining the signals, and a discrimination unit 6c for discriminating data using the combined signal.
The reception level of the signal component (the desired signal) of the local channel sent from a transmitter varies in dependence upon multipath, as shown in FIG. 29, and arrival time at the receiver differs as well. Further, the signal received at the antenna includes other channel components in addition to the channel that has been assigned to the local receiver. Accordingly, the searcher 5 extracts the desired signal from the antenna receive signal by a matched filter (not shown) using the spreading code of its own channel. That is, when a direct-sequence signal (DS signal) that has been influenced by multipath is input to the search 5, the latter performs an autocorrelation operation using the matched filter and outputs a pulse train having a plurality of peaks. The searcher detects multipath from the pulse train based upon peak signals MP1 to MP4 that are larger than a threshold value, detects delay times t1 to t4 on each of the paths and inputs despreading-start timing data and delay-time adjustment data to the fingers 6a1 to 6az corresponding to the respective paths.
A despreader/delay-time adjusting unit 7 of each of the fingers 6a1 to 6az subjects a delayed wave that arrives via a prescribed path to despread processing using a code identical with the spreading code, performs dump integration, then applies delay processing conforming to the path and inputs the processed signal to a synchronous detector (phase correction unit) 8. The synchronous detector 8 detects a pilot signal contained in the receive signal, finds the phase difference between this pilot signal and an already known pilot signal, and restores the phase of the despread information signal by the amount of the phase difference. The information signal and pilot signal contained in the receive signal both undergo phase rotation owing to transmission. However, if a signal-point position vector PACT (see FIG. 30) of this pilot signal is known on the receiving side, then the phase-rotation angle θ of the pilot signal ascribable to transmission will be obtained because the ideal signal-point position vector PIDL is already known. Accordingly, the synchronous detector 8 detects the pilot signal, calculates the phase-rotation angle θ thereof and subjects each information signal to processing to rotate it by a rotation angle of −θ, thereby restoring the original information signal. As a result, the discrimination unit 6c is capable of performing highly precise demodulation of data.
The signals output from the phase detectors 8 of respective ones of the fingers 61a to 6az are combined upon being weighted in accordance with the reception level, and the discrimination unit 6c discriminates “1”, “0” of the data using the combined signal.
FIG. 31 is a diagram showing another structure of the CDMA receiver, in which components identical with those of the CDMA receiver of FIG. 28 are designated by like reference characters. This structure differs in that (1) a valid-path detector 5a is provided within the searcher to detect whether a path is valid or invalid and input the result of the path valid/invalid detection to the RAKE combiner 6b, and (2) only output signals from fingers conforming to the valid paths are weight in accordance with the signal level and combined by the RAKE combiner 6b. FIG. 32 is a diagram showing the structure of the valid-path detector. Average power calculation units 5b1 to 5bN calculate first to Nth average powers based upon the outputs of matched filters (not shown), and a power comparator 5c obtains maximum and minimum average powers Pmax, Pmin, recognizes valid paths that satisfy the following expressions:Pmax≧P>(Pmax−N) P≧Pmin+M and invalid paths that do not satisfy these expressions, and reports these paths to the RAKE combiner 6b. 
With the RAKE receiver of FIG. 28, all paths are combined as valid paths by the RAKE combiner and data discrimination is performed based upon the combined signal. However, the paths include invalid paths and the invalid paths constitute interference. In other words, in accordance with the RAKE receiver of FIG. 28, even invalid signals are combined, a decline in sensitivity is produced and receiver performance is affected.
On the other hand, with the RAKE receiver of FIG. 31, path validity/invalidity is detected and only valid paths are combined. As a result, sensitivity and performance are improved in comparison with the RAKE receiver of FIG. 28. It should be noted that a base station performs transmission power control in order to render reception power constant (in order to render reception quality at the base station constant). As a result, reception power of direct waves or delayed waves shown in (a) of FIG. 33 declines in accordance with transmission power control, as shown in (b) of FIG. 33, and therefore it becomes difficult to distinguish between valid paths and invalid paths. Further, since all paths are not necessarily valid paths (paths on which gain is obtained), characteristics deteriorate if all of these paths are combined. In other words, with a conventional RAKE receiver, there are instances where invalid paths are regarded as valid paths and combined and instances where valid paths are regarded as invalid paths and are not combined. A problem that results in a decline in sensitivity and performance.
Further, with the conventional RAKE receiver, a problem which arises is the need to provide the highly precise valid-path detector circuit, which relies upon the setting of two-stage threshold values, as mentioned above, in order to detect invalid paths without causing a decline in sensitivity.
Further, in a CDMA receiver, it is necessary to change the valid/invalid-path discrimination processing or parameters depending upon the transmission rate. However, the conventional RAKE receiver does not possess such a function and, hence, cannot cope with transmission rate.